That’s right, D can also help you get stronger, increase muscle protein synthesis, reduce bodyfat and raise testosterone levels. FLEX recommends getting in a good 2,000–5,000 international units of vitamin D per day, but not just any ol’ D will do. You need to make sure you are taking vitamin D3, known as cholecalciferol. In a recent study from Creighton University (Omaha, Nebraska) subjects took approximately 7,000 IU of vitamin D2 or vitamin D3 per day for 12 weeks. They reported in a 2011 issue of the Journal of Clinical Endocrinology & Metabolism that D3 was about 90% more potent at raising levels of 25-hydroxyvitamin D, the storage form of vitamin D in the body, than vitamin D2. It also increased stored levels of the vitamin three times more than D2 did.